It is known in the art that an alkylate can be produced from olefins and isoparaffins using HF as an alkylation catalyst. The octane number of this alkylate can be further improved by carrying out the alkylation process in the presence of ethyl fluoride as a promoter. Ethyl fluoride is either produced in the alkylation reactor by introducing ethylene into the reactor containing HF or it is added separately to the alkylation reactor. Since the HF catalyst and the ethyl fluoride are expensive compounds, it would be desirable to have a process available which readily recovers ethyl fluoride from the system so that it can be used as the promoter, e.g. in cases where no ethylene is available or in cases where it is desirable to add ethyl fluoride to compensate for losses of ethyl fluoride in the process of producing alkylates.